The present invention relates to an image processing system for preparing a mask used when cutting out an image of a predetermined object from an image and a mask preparation method and program for the same.
The present invention for example relates to an image processing system for preparing a mask used when cutting out an image of a biometric feature from an image captured for biometric authentication.
Along with the advances made in information communications utilizing networks, a personal authentication system having a higher safety is demanded.
Biometric authentication judges whether or not a person is a true registered person based on information obtained from physical characteristics of the person, for example, a fingerprint, voice print, retinal pattern, and vein pattern of a finger, therefore has the excellent characteristic that another person assuming the identity of a true person can be much reduced.
Japanese Patent Publication (A) No. 2004-329825 discloses a system for authenticating a person by emitting near-infrared light, capturing the image of a pattern of blood vessels of the finger, and comparing this with a previously registered pattern of the blood vessels.
When authenticating a person based on a blood vessel pattern, in general, processing a captured image by a suitable image filter makes the outer shape of the blood vessels stand out and facilitates the authentication of their characteristics. At this time, portions unnecessary for the authentication, for example, the system, background, etc. similarly end up standing out, therefore, there is a possibility of erroneous authentication of such unnecessary portions as blood vessels as they are.
Therefore, usually masking is performed for cutting out only an object necessary for authentication from a captured image.
The system disclosed in Japanese Patent Publication (A) No. 2005-56282 prepares a histogram representing the number of pixels having different pixel values included in a captured image, acquiring a threshold value for differentiating an object and the background portion based on this histogram, and using this threshold value to binarize the captured image to thereby prepare a mask.
In general, in a method using a threshold value to binarize an image, the threshold value for suitably cutting out an object changes according to the intensity of the rays irradiated to the object, the brightness of the background, the light transmittance of the object, and other conditions. For this reason, setting the threshold value in accordance with the captured image becomes necessary and the processing becomes complicated as in the system disclosed in Japanese Patent Publication (A) No. 2005-56282.